Method for reading data from nonvolatile storage element, and nonvolatile storage device

Abstract

Provided is a method for reading data from a variable resistance nonvolatile storage element, where the operation for reading data is less susceptible to a fluctuation phenomenon of resistance values in reading the data. The method includes: detecting a current value I read that flows through the nonvolatile storage element that can be in a low resistance state RL and a high resistance state RH, with application of a fixed voltage; and determining that (i) the nonvolatile storage element is in a high resistance state when the current value I read detected in the detecting is smaller than a current reference level Iref, and (ii) the nonvolatile storage element is in a low resistance state when the current value I read detected in the detecting is larger than the reference level Iref, the current reference level Iref being defined by (IRL+IRH)/2<Iref<IRL.

The invention claimed is: 
     
       1. A method for reading data from a variable resistance nonvolatile storage element (i) including a first electrode, a second electrode, and a variable resistance layer disposed between and in contact with the first electrode and the second electrode and (ii) having characteristics in which a resistance state between the first electrode and the second electrode with application of a voltage having a first polarity between the first electrode and the second electrode becomes a first resistance state RL, and in which the resistance state between the first electrode and the second electrode with application of a voltage having a second polarity different from the first polarity between the first electrode and the second electrode becomes a second resistance state RH, the second resistance state RH> the first resistance state RL, the nonvolatile storage element being an element having fluctuations that are characteristics in which a resistance value of the nonvolatile storage element in the second resistance state RH randomly changes with passage of time, the method comprising:
 detecting a current that flows through the nonvolatile storage element with application of a fixed voltage; and 
 determining that (i) the nonvolatile storage element is in a high resistance state when the current detected in the detecting is smaller than a current reference level Iref, and (ii) the nonvolatile storage element is in a low resistance state when the current detected in the detecting is larger than the reference level Iref, the current reference level Iref being defined by (IRL+IRH)/2<Iref<IRL, where IRL denotes a current that flows through the nonvolatile storage element in the first resistance state RL with application of the fixed voltage, IRH denotes a current that flows through the nonvolatile storage element in the second resistance state RH, and IRH<IRL. 
 
     
     
       2. The method according to  claim 1 ,
 wherein in the determining, a current value larger than an average value of the fluctuations by at least 4σ is determined as the current reference level Iref satisfying (IRL+IRH)/2<Iref<IRL, where σ denotes a standard deviation in the fluctuations of the current value IRH of the nonvolatile storage element in the second resistance state RH. 
 
     
     
       3. The method according to  claim 1 ,
 wherein the variable resistance layer has a stacked structure including (i) a first transition metal oxide comprising a first transition metal and (ii) a second transition metal oxide comprising a second transition metal, the first transition metal oxide being higher in oxygen deficiency than the second transition metal oxide. 
 
     
     
       4. The method according to  claim 3 ,
 wherein the second transition metal oxide is larger in resistance value than the first transition metal oxide. 
 
     
     
       5. The method according to  claim 3 ,
 wherein the first transition metal oxide is identical to the second transition metal oxide. 
 
     
     
       6. The method according to  claim 5 ,
 wherein the first transition metal oxide and the second transition metal oxide comprise tantalum. 
 
     
     
       7. The method according to  claim 3 ,
 wherein the first transition metal oxide is different from the second transition metal oxide, and 
 the second transition metal oxide is lower in standard electrode potential than the first transition metal oxide. 
 
     
     
       8. A nonvolatile storage device, comprising:
 a variable resistance nonvolatile storage element; and 
 a control unit configured to read data from the nonvolatile storage element,
 wherein the nonvolatile storage element (i) includes a first electrode, a second electrode, and a variable resistance layer disposed between and in contact with the first electrode and the second electrode and (ii) has characteristics in which a resistance state between the first electrode and the second electrode with application of a voltage having a first polarity between the first electrode and the second electrode becomes a first resistance state RL, and in which the resistance state between the first electrode and the second electrode with application of a voltage having a second polarity different from the first polarity between the first electrode and the second electrode becomes a second resistance state RH, the second resistance state RH> the first resistance state RL, the nonvolatile storage element being an element having fluctuations that are characteristics in which a resistance value of the nonvolatile storage element in the second resistance state RH randomly changes with passage of time, and 
 
 the control unit is configured to: 
 detect a current that flows through the nonvolatile storage element with application of a fixed voltage; and 
 determine that (i) the nonvolatile storage element is in a high resistance state when the detected current is smaller than a current reference level Iref, and (ii) the nonvolatile storage element is in a low resistance state when the detected current is larger than the reference level Iref, the current reference level Iref being defined by (IRL+IRH)/2<Iref<IRL, where IRL denotes a current that flows through the nonvolatile storage element in the first resistance state RL with application of the fixed voltage, IRH denotes a current that flows through the nonvolatile storage element in the second resistance state RH, and IRH<IRL. 
 
     
     
       9. The nonvolatile storage device according to  claim 8 ,
 wherein the control unit is configured to determine a current value larger than an average value of the fluctuations by at least 4σ as the current reference level Iref satisfying (IRL+IRH)/2<Iref<IRL, where σ denotes a standard deviation in the fluctuations of the current value IRH of the nonvolatile storage element in the second resistance state RH. 
 
     
     
       10. The nonvolatile storage device according to  claim 8 ,
 wherein the variable resistance layer has a stacked structure including (i) a first transition metal oxide comprising a first transition metal and (ii) a second transition metal oxide comprising a second transition metal, the first transition metal oxide being higher in oxygen deficiency than the second transition metal oxide. 
 
     
     
       11. The nonvolatile storage device according to  claim 10 ,
 wherein the second transition metal oxide is larger in resistance value than the first transition metal oxide. 
 
     
     
       12. The nonvolatile storage device according to  claim 10 ,
 wherein the first transition metal oxide is identical to the second transition metal oxide. 
 
     
     
       13. The nonvolatile storage device according to  claim 12 ,
 wherein the first transition metal oxide and the second transition metal oxide comprise tantalum. 
 
     
     
       14. The nonvolatile storage device according to  claim 10 ,
 wherein the first transition metal oxide is different from the second transition metal oxide, and 
 the second transition metal oxide is lower in standard electrode potential than the first transition metal oxide. 
 
     
     
       15. The method according to  claim 1 ,
 wherein the nonvolatile storage element has fluctuations that are characteristics in which a resistance value of the nonvolatile storage element in the first resistance state RL randomly changes with passage of time; and 
 the nonvolatile storage element in the second resistance state RH has the fluctuations in resistance value larger than the fluctuations of the nonvolatile storage element in the first resistance state RL. 
 
     
     
       16. The nonvolatile storage device according to  claim 8 ,
 wherein the nonvolatile storage element has fluctuations that are characteristics in which a resistance value of the nonvolatile storage element in the first resistance state RL randomly changes with passage of time; and 
 the nonvolatile storage element in the second resistance state RH has the fluctuations in resistance value larger than the fluctuations of the nonvolatile storage element in the first resistance state RL.